A high-speed environment, in which the user equipment (UE) moves at speeds of two hundred kilometers per hour or more, poses a great challenge for mobile communications. The Doppler Effect and quick handover are the toughest problems to deal with. The higher the speed is, the worse the effects are.
When the UE is in movement, the frequency of signals at the receiver end will change. This is commonly known as the Doppler Effect. The frequency change is referred to as the Doppler shift and its effect is observed on both the NodeB and the UE. The Doppler shift can degrade communication quality, and even induce service interruption. The Doppler Effect is especially cumbersome in a high-speed environment.
In modulation coherence adopted by the NodeB, the carrier used for demodulation must be operating on the same frequency and phase as that of the signal carrier. Frequency shift caused by the Doppler Effect, which, in some cases, may be as much as 750 Hz in the uplink, clearly has a deleterious effect on demodulation performance and further adversely affects service access, system capacity and coverage.